1. Field of the Invention
The present invention relates generally to power amplifier circuits, and more particularly, to methods and systems for controlling the output power of a power amplifier circuit.
2. Description of the Related Art
Portable transmitting devices are becoming more and more common, led first by the popularity of cellular telephones and increasingly by wireless networking products such as under the “Bluetooth” and IEEE 802.11 type (802.11) specifications. Two requirements of these types of products is a small physical size and efficient, long term (e.g., multiple days') operation relying on only battery power source. Another important requirement is that they provide minimal interference with other, similar devices that may be nearby.
Often these types of transmitters are used in very close proximity with each other and their corresponding receivers. By way of example, an 802.11 transceiver (receiver/transmitter combination) typically has a maximum range of up to about 100 meters or more to a corresponding 802.11 transceiver. However, 802.11 transceivers can also be used in very close proximity such as with only one meter or two.
Devices that contain one or more of these types of transmitters must be very power efficient. The need is driven by the low cost, small size, light weight and minimal battery capacity requirements of such competitive portable devices (e.g., cellular telephones, personal digital assistants, headsets, etc.).
Examples of areas of power efficiency gains include adjusting the output power of the transmitters. By way of example, the output power of an 802.11 transmitter needs to be adjustable for the specific conditions of use (e.g., location, proximity to corresponding receiver). Ideally, the power output should be adjustable over a selected power range and with a selected resolution. Another aspect of the power efficiency gains that are possible in portable transmitters include minimizing the power consumption such as by ensuring the transmitter circuitry is as power efficient as possible.
Wireless communication devices inherently share frequency bands and typically several devices may be transmitting simultaneously on neighboring channels. The sharing of frequency bands drives the requirement that each transmitter not interfere with other transmitters such as minimizing the emitted energy outside the selected channel. An important source of unwanted emissions is an abrupt change in output power level. When the transmitted power output abruptly transitions either upward or downward, undesirable harmonics of the selected channel frequency can be emitted. The undesirable harmonics can interfere with other channels in the frequency band. By way of example, if a transmitter is abruptly turned off (or on), undesirable harmonics can be emitted. In view of the foregoing, there is a need for systems and methods of efficiently adjusting the power output of a portable transmitter in a manner that substantially minimizes emission of undesirable harmonics.